Rigid floor coverings such as tile require a sturdy and stable base for longevity. Because wood structures expand and contract with changes in humidity—and it is inherently flexible, wood it is not ideal for tile floors that are intended to have an appreciable lifespan. Many of today's building are constructed using an engineered wood particle board such as oriented strand board (OSB). Tile and thinset mortar producers belonging to the Tile Council of North America do not recommend installing ceramic tile directly to OSB or over other single-layer plywood floor systems. The ANSI standards for tile installation (A108) require subfloor rigidity that can only be provided by a layer of cement board or a membrane component. The primary method for stiffening wood floors used today is to screw sections of ½″ thick cement board to the subfloor and patch the seams with thinset mortar.
Concrete subfloors possess the required rigidity to support tile flooring but have other problems including lateral forces caused by shifting, settling, or fracturing concrete. As such, the Tile Council of North America recommends bonding a crack isolation/anti-fracture membrane to the concrete subfloor prior to installing tile. The tile is then bonded with an adhesive mortar such as thinset mortar to the top surface of the membrane. The membrane acts to prevent movement in the concrete from being directly transferred to the tile flooring. Typically, membranes are vacuum-formed high density polyethylene plastic with an anchoring fleece laminated to the bottom surface.